Which plants store more carbon in Australia: forests or grasses?

Since carbon dioxide is an important greenhouse gas, one strategy that can partially combat global warming and climate change is to increase the amount of carbon stored in plants. By increasing the amount of plant life on earth, or altering it to plant types that store the most carbon, more carbon dioxide may be pulled out of the air and stored for a period of time.

Scientists call anything that removes carbon from the atmosphere a ‘sink’. In order to be effective in combating climate change, the sink must be large and the carbon must stay in the sink. So what is important for climate change is not the amount of carbon exchanged between the atmosphere and plants, but how much carbon stays in the total forest and total grassland ‘sinks’.

Australia has 149 million hectares of forest. Of this, 147 million hectares is native forest, dominated by eucalypt (79%) and acacia (7%), and 1.82 million hectares is in plantations[i]. Grassland covers around 440 million hectares of land in Australia[ii].

The size of the difference in the total carbon storage between grasslands and woodlands depends not just on the amount of land covered by the plants, but on the capacity of the individual ecosystems to store carbon, and the depth to which the carbon sink is tested. The sinks can be the plant material above ground, below ground (roots), and soil that is enriched in carbon by dead plant material.

Based on data from typical perennial grasslands and mature forests in Australia, forests are typically more than 10 times as effective as grasslands at storing carbon on a hectare per hectare basis.

Carbon in plants

Carbon is continuously exchanged between various elements of the earth: atmosphere, soil, ocean and life, which is predominately plant material. The length of time it takes for the carbon to be exchanged depends on the process involved. In the process known as photosynthesis, plants generate their own ‘food’ by absorbing carbon dioxide (CO2), water (H2O) and sunlight to create sugars. Excess oxygen is released, and carbon is stored in the sugars and starches (particular combinations of carbon (C), hydrogen (H) and oxygen (O) in the plant material.

Forests

The amount of carbon taken up every year by dry forests in Australia depends on the weather conditions and age of the trees. Science tells us that the range for forests with continuous canopies is about 0.5-2 tonnes of carbon per year for each hectare. Grasslands may have a similar annual rate of net carbon uptake[i], but the long-term storage of carbon per hectare of grasslands is less than that over an average hectare in woody trees.

In other words, over the long haul, more carbon stays in the tree sink than in the grass sink. Some Australian native eucalyptus forests store up to ten times more carbon per hectare than Australian native and introduced grasslands – both above and below ground[ii].

The Co-operative Research Centre for Greenhouse Accounting has estimated that Australian forests store about 10.5 billion tonnes of carbon (excluding soil carbon)[iii]. This store of solid carbon has accumulated over an assumed life of 100 years for native eucalypt regrowth. That translates to our forests storing an amount of carbon equivalent to almost 38.5 billion tonnes of gaseous carbon dioxide from the atmosphere, about 70 times Australia’s annual net greenhouse gas emission.

Grasses

Using data from a study of semi-arid Australian grasslands by the Queensland Department of Primary Industry[iv] that accounted for the amount of live grass above ground found that about 5 tonnes of carbon could be stored per hectare of perennial grass year, assuming little grazing. This compares to carbon stocks of mature dry sclerophyll forest that contain about 100 tonnes of carbon per hectare (with wide variability). A recent ANU study assembling data from Australia’s unlogged, natural eucalypt forests concluded that kind of ecosystem may even hold an average of 640 tonnes of carbon per hectare[v].

So, in order for grasslands to have a greater carbon stock than an equivalent acreage of Australian forest, the roots of a summer pasture grass such as kangaroo grass, panic or weeping grass, would have to contain more than 10 times the mass of the grass that you can see above the ground[vi], which is not the case.

Soil carbon

Carbon can also be stored in the soil itself in the form of old organic matter. Depending on the depth of soil investigated, the nutrient level of the soil and the availability of water, grassland soil can have either a similar or much lower amount of carbon than does the soil beneath forests.

As an example, studies done in 1999[vii] and again in 2005 show that reducing the amount of tree cover tends to decrease the amount of organic carbon in deep soil sinks[viii]. The 2005 study showed that about 1 metre underground, grassland sites contained only 25 tonnes of carbon in the soil per hectare compared with the soil in treed savannah sites, which stored 30 to 70 tonnes per hectare.

The NSW Department of Primary Industry has compared soil organic carbon under perennial pasture in high rainfall areas in the mid-north coast of NSW to native hardwood forests within a 100km radius. They found that for the high-rainfall areas studied, there was no significant difference between soil organic carbon in the pastures and native forests at 20 centimetres depth, with an average storage of 72.9 tonnes per hectare in the pasture versus 76.5 tonnes per hectare in the native forest sites[ix].

[iii] Ximenes F, Robinson M, and Wright B, (2007) Forests, Wood and Australia’s carbon balance, Australian Government Forest and Wood Products Research and Development Corporation and Cooperative Research Centre for Greenhouse Accounting , http://www.plantations2020.com.au/assets/acrobat/Forests,Wood&CarbonBalance.pdf